The present invention relates generally to portable and handheld radiation detection devices. More particularly, this invention pertains to wipe test plates which are used to position a radiation monitoring wipe or disk proximate the detector on a portable radiation monitor.
Portable and handheld radiation monitoring instruments are used in a wide number of diverse applications to measure the level of alpha, beta, gamma, and x-ray radiation in the proximity of the instrument. Persons working in occupations where they are possibly exposed to radiation from medical and environmental hazards are required to monitor for radioactive surface contamination to ensure the safety of personnel, determine if their exposure is within established limits, and prevent the spread of contamination. A typical art approach to monitoring occupational exposure to radiation from surface contamination is to have a worker perform a smear or "wipe" test. This test consists of wiping a surface to be checked, such as a countertop, floor, equipment, etc. with a wipe which is typically a circular disc of cotton or filter paper and then measuring the accumulation of any radioactivity accumulated on or attached to the wipe. This typically small amount of radioactivity must have a prolonged measurement by a radiation monitor.
It is convenient to use a portable handheld GM (Geiger-Mueller) type detector to perform this measurement. The GM type that is required must have a very thin circular glass window. The wipe should be placed directly adjacent to the window of the GM while an accumulating count is performed over a chosen time period. The final measurement is determined by a calculation of the reading from the instrument, the distance of the instrument from the wipe, and the measurement time period.
The distance or geometry of the wipe to the GM window and the positioning of the wipe precisely adjacent to the window is critical. For example, if the wipe is positioned so a portion of the wipe is not in a direct line to the window, the radioactivity contained on the part of the wipe that is not aligned with the window may not be detected and may give an artificially low reading on the instrument. In addition, if the distance from the window to the wipe is only estimated, this also can result in a miscalculation and incorrect final measurement.
Unfortunately, there has been no design developed in the prior art that allows the owner of a portable or handheld radiation monitoring instrument to easily and consistently correctly position a wipe adjacent to the GM window on the instrument. Consequently, the user of the instrument must carefully manipulate the wipe trying to visualize and estimate the position and distance of the wipe to the window, a procedure that can be time consuming and inaccurate and produce radiation exposure risks to personnel.
What is needed, then, is device that assists the user of a portable radiation monitor in accurately and consistently positioning a wipe adjacent and at a known distance to the GM window on the instrument without significantly adding to the cost or complexity of the instrument.